In data processing systems, magnetic disc memory systems are frequently used because they have large storage capacity and require a relatively short time for a magnetic read/write head to access data contained anywhere on discs of the memory from the moment the head receives an instruction to access the data. The magnetic discs are driven at constant rotational velocity by an electric motor.
A magnetic disc carries data, usually in binary form, on both faces of the disc in concentric circular recording tracks having a width that does not exceed a few hundredths of a millimeter. The tracks are identified by allotting them an address or serial number j (j being a whole number) from 0 to (N-1), where N is the total number of recording tracks. The tracks having addresses (j-1) and (j+1) are adjacent tracks j.
Memories having a small storage capacity contain a limited number of discs (normally only one or two). In small memories, data are stored, i.e., recorded, on each of the disc faces by setting a large amount of space aside for the data intended to be processed by the data processing system of which the memories are a part. A small amount of space is set aside for data that enables the tracks to be located; these data indicate the track addresses and enable the magnetic head to be servo-controlled to a position above the tracks. In the small area are also stored data which indicate whether or not the tracks contain faults.
For the sake of simplicity, a memory is considered which contains only a single disc. Preferably, each face of the disc is associated with a single magnetic read/write head, i.e., a magnetic read/write transducer. Current practice, as described in commonly assigned U.S. patent applicaton Ser. No. 76,332, filed on Sept. 17, 1979, entitled "Method Of Writing Information On A Magnetic Recording Medium", now U.S. Pat. No. 4,354,208, issued Oct. 12, 1982, is for the data contained on each face of the disc to be distributed over equal adjacent circular sectors S.sub.0, S.sub.1 . . . S.sub.i . . . S.sub.n. Generally, one face of a disc is divided into several tens of sectors (usually forty to fifty).
When the magnetic disc face associated with the magnetic head passes in front of the head, sectors S.sub.0, S.sub.1, S.sub.2, etcetera, are read by the head in sequence. It is therefore said that sector S.sub.0 precedes sector S.sub.1, that sector S.sub.1 precedes sector S.sub.2, that sector S.sub.i precedes sector S.sub.i+1 and so on. In more general terms, if two items of information I.sub.k-1 and I.sub.k which follow one another along the same track j on the face are considered, item I.sub.k-1 precedes item I.sub.k if item I.sub.k-1 is read by the head before I.sub.k, or that item I.sub.k follows item I.sub.k-1. The same reasoning is applied to groups of information items G.sub.k and G.sub.k-1 in a track (j+1) adjacent and abutting with track j.
Each sector S.sub.i is divided into two unequal areas. The larger area contains the data to be processed by the data processing system of which the disc memory is a part, while the smaller area contains data for locating the tracks and indicating faults. For the sake of simplicity, the data contained in the larger area are referred to as "data to be processed". In each sector, the smaller area is divided into a plurality of reference zones, one for each track, so each track is associated with a single reference zone.
It is recalled that a bit is a binary one or zero digit. The one or zero may be expressed on a magnetic medium or as an analog or logic electrical signal. A logic signal is capable of assuming only two values called "logic or binary zero" and "logic or binary one"; an analog signal is a signal having a voltage that may vary continuously between two positive and/or negative extreme values. Any item of data or information recorded on the disc is referred to herein as a "bit".
A magnetic head for writing information into and reading information from a magnetic disc includes a magnetic circuit comprising a high magnetic permeability material on which is mounted a winding and in which is formed an air gap. The air gap is substantially rectangular in shape, having a length much greater than its width. The gap is of the same order of magnitude as the radial width of the tracks and reference zones, which are of the same width. Thereby, the gap is responsive to magnetic flux variations representing data to be processed from a disc track have serial number j, as well as track identifying data contained in reference zones ZRP.sub.ij and ZRP.sub.i(j+1) associated with the data track having serial number j. The air gap of the head is disposed perpendicularly to magnetic axis Ax.sub.j of track j, i.e., the air gap is disposed parallel to the radial width of track j. To enable the data of track j to be read from the disc or written into the disc with maximum accuracy, the head remains stationary facing the track during the time necessary for reading or writing all or part of the data which the track contains while the disc rotates at constant velocity. The head air gap is perfectly centered on magnetic axis Ax.sub.j, the boundary between reference zones ZRP.sub.ij and ZRP.sub.i(j+1). The magnetic read/write head reads or writes track identifying data in reference zones ZRP.sub.ij and ZRP.sub.i(j+1) by being disposed astride the magnetic axis separating the two reference zones.
One known method of recording data on the face of a magnetic disc involves providing a succession of elementary areas of variable length over the entire length of each track and each zone by applying magnetic fluxes to the zones by the magnetic head. Alternate areas have magnetic inductions of the same amplitude, but of opposite polarity, whereby, for example, a first area has a magnetization of +.phi. and the adjacent area has a magnetization of -.phi.. The boundary between two adjacent magnetic areas which follow one another along a track or zone defines a magnetization sense change or a "magnetic transition".
There are two different types of magnetic transitions, namely: when the magnetic head passes successive magnetic areas having negative and positive induction on the disc, the magnetization sense change is positive; and, when, on the other hand, the head passes successive areas having positive and negative induction, the magnetization sense change is negative.
A preferred configuration for magnetic induction values of track identification data contained in the reference zones is described and claimed in previously mentioned U.S. Application Ser. No. 76,332. In the disc disclosed in said application, track identifying data in a reference zone is defined by the presence or absence of a pair of magnetic transitions along the length of the cell. If two such transitions occur within the cell, the first transition is of opposite polarity to the second transition to represent a first binary value. The second binary value is represented by no magnetic transitions along the length of the cell. The position data for each reference zone is contained in a part of the reference zone designated as PPOS. The nomenclature is such that a particular reference zone ZRP.sub.ij contains position data in part PPOS.sub.ij. Part PPOS.sub.ij is preceded by another part of the reference zone that contains the address of track j.
Part PPOS contains plural (m) successive cells, each having the same length, such that alternate cells contain a double magnetic transition and intermediate cells contain no such transitions. The positional data are written identically into even and odd numbered reference zones whereby the positional data is shifted from one cell to another. Thus, for any even numbered reference zone, the cells of odd rank, i.e., the odd numbered cells, do not include a double transition while the even numbered cells do contain such a transition. In all odd numbered reference zones, the odd numbered cells contain a double transition, but the even numbered cells do not contain any transitions.
Because the even and odd numbered cells of part PPOS of any reference zone have the same length, the time that the head is over each cell in a reference zone is identical, and designated by T. In response to the head traversing part PPOS of an even numbered reference zone such that the head air gap is perfectly centered on part PPOS, the head derives a periodic output signal S.sub.p having a period P equal to 2T. During each period of the head output, the head output signal has a zero voltage during a first half-period, which occurs as the head moves past the odd numbered cells. During the second half-period, as the head moves past even numbered cells, the head output is composed of two opposite polarity pulses having substantially the same absolute magnitude AMP. In response to the air gap of the head being completely centered on part PPOS of an odd numbered reference zone, the head derives a periodic output signal S.sub.imp having a period P equal to 2T. During each period, signal S.sub.imp is composed of two analog pulses of opposite sign but of equal absolute magnitude AMP during the first half-period, i.e., as the head moves past the odd cells. During the second half-period of S.sub.imp, while the head moves past even numbered cells within the odd numbered reference zone, the head derives a zero output signal. Thus, signals S.sub.p and S.sub.imp are respectively referred to as even and odd signals. Signals S.sub.p and S.sub. imp are thus periodic, having the same period P, and shifted in time by a half-period.
The following nomenclature is employed in the present specification: if two adjacent reference zones ZRP.sub.ij and ZRP.sub.i(j-1) are considered, either one of which may be even and the other odd, reference zone ZRP.sub.ij is considered to be closest to the center of the disc and to have a track number j larger than reference zone ZRP.sub.i(j-1) ; POS.sub.1 designates the position of a magnetic read/write head such that the air gap is situated entirely over part PPOS.sub.ij of zone ZRP.sub.ij, i.e., the air gap is perfectly centered on part PPOS.sub.ij of zone ZRP.sub.ij ; POS.sub.3 designates the position of the magnetic head such that the head air gap is situated entirely over part PPOS.sub.i(j+1) of zone ZRP.sub.i(j-1), i.e., the head air gap is perfectly centered on part PPOS.sub.i(j+1) ; an axis between zones ZRP.sub.ij and ZRP.sub.i(j-1) is defined as Ax.sub.j ; when the head air gap occupies any location between the extreme positions POS.sub.1 and POS.sub.3, portion X.sub.1 of the air gap faces the even reference zone, while portion X.sub.2 of the air gap faces the odd reference zone; thus X.sub.1 and X.sub.2 are fractions between zero and one and X.sub.1 +X.sub.2 =1.
Because the head air gap is of substantially rectangular shape, the value of X.sub.1 and X.sub.2 is proportional to the air gap length L, in turn substantially equal to the radial width of a track. Thus, if three-quarters of the air gap length is situated above an even reference zone, X.sub.1 =3/4=0.75 and X.sub.2 =(13/4)=0.25. Signal S.sub.p derived from the head as it traverses an even numbered reference zone can be considered as the algebraic sum of an even part S.sub.1 and an odd part S.sub.2. Even part S.sub.1 corresponds to the signals resulting from reading the positional data of the even zone which the head is traversing, while odd part S.sub.2 corresponds to the signals read from the positional data of the odd zone as the head moves past portion X.sub.2. For a given data item, the voltage derived from the read head varies linearly as a function of the value of X. Therefore, S.sub.1 =X.sub.1 (S.sub.p) and S.sub.2 =X.sub.2 (S.sub.imp). Consequently, S.sub.T =S.sub.1 +S.sub.2 =X.sub.1 (S.sub.p)+X.sub.2 (S.sub.imp), where S.sub.T =the total output signal of the head. Thus, signals S.sub.1 and S.sub.2 are the same shape and have the same period P, but are shifted by one half-period (P/2) in time relative to each other.
Each period P of signal S.sub.T thus comprises:
(1) a half-period signal S.sub.1 including two analog pulses of opposite polarity and equal absolute amplitude value, X.sub.1 (AMP); and
(2) a half-period odd signal part S.sub.2 composed of two opposite polarity analog pulses having equal absolute values equal to X.sub.2.
Because the average of signal S.sub.T over a complete period, as well as over an integral number of periods, is zero, the calculated integral over one period of over an integral number of periods is zero. Therefore, the location of the magnetic head from position POS.sub.1 to POS.sub.3 cannot be deduced by calculating the integral of signal S.sub.T or the average value of the voltage resulting from signal S.sub.T.
A prior art device, disclosed in copending application, Ser. No. 186,294, filed Sept. 11, 1980, entitled "Apparatus And Method For Displacing A Movable System With Respect To A Data Carrier", and commonly assigned with the present application, enables a head of a disc memory to be displaced in a minimum possible time between a departure track and an arrival track of serial number j from which it is desired to read data to be processed. In the system disclosed in said application, when the head has been positioned with respect to axis Ax.sub.j of track j, it is necessary to keep the head astride or centered on axis Ax.sub.j so that the data read by the head from the track can be performed with maximum accuracy. By maintaining the head centered on the axis, the amplitude of signals derived from the head is maximum during the entire time while data are read from the track. Therefore, the air gap is perfectly centered on the axis, whereby X.sub.1 =X.sub.2 =0.5, a position designated as POS.sub.2. Separations of the air gap from position POS.sub. 2 toward position POS.sub.1 or position POS.sub.3 result in greater risks of error in reading or writing data to be processed from the disc.
It is therefore extremely important to be able to precisely determine the position the head occupies, i.e., the head air gap position, from position POS.sub.1 to position POS.sub.3 as the head is being positioned by the displacement device on track j. When a boundary between two parts PPOS.sub.ij and PPOS.sub.i(j-1) of adjacent even and odd zones moves past the head air gap, while the head occupies any position from POS.sub.1 to POS.sub.3, the head or its air gap is considered as being disposed "in the neighborhood of the boundary".
It is, accordingly, an object of the present invention to provide a new and improved method of and apparatus for measuring the position of a transducer head for reading and/or writing data on a carrier with respect to a reference position of the carrier.
Another object of the invention is to provide a new and improved method of and apparatus for deriving an analog signal defining the actual position occupied by an air gap of a magnetic head reading data from a magnetic memory element wherein the head position is determined with respect to a reference position defined by a boundary between adjacent even and odd zones of the memory element.
An additional object of the invention is to provide a new and improved apparatus for and method of deriving analog data indicative of the position of a magnetic head relative to an axis between a pair of reference zones of a magnetic disc wherein the analog signal is adapted to be transmitted to a servo-control device that maintains the head astride the axis during the entire time while data to be processed are read from a data sector associated with the axis.